I'd persevere a little with your C4 before throwing away a lot of money on a change. Firstly, it should not be underpowered for overtaking, though maybe the SCR Adblue system is taking a toll. I'd try it on Shell V-Power Nitro Plus. By the second tankful you should see an improvement both in power and in fuel economy. As for the comfort, your wife will relieve pressure on crushed vertebrae by sitting in the seat with the backrest slightly reclined. This has the effect of feeding some of the weight of her torso into the backrest rather than into the base of her spine. Sitting upright is the worst thing she can do. (This is from professor Mark Porter, the top UK vehicle ergonomist.)